The invention relates to a method for the transfer of a digital data signal with predetermined bandwidth from a transmitter to a receiver using the spread spectrum technique, in which the data signal is modulo-2 added with a PN code sequence, the bit rate of which is very much higher than the bit rate of the data signal, then a carrier is modulated with the thereby resulting spread data signal, the modulated carrier is transmitted, in the receiver the modulated carrier is demodulated and the demodulated signal, to obtain the digital data signal, is multiplied with a PN code sequence produced in the receiver, synchronized with the PN code sequence of the transmitter through autocorrelation.
Data transfer methods which operate using the spread spectrum technique are used especially when a high suppression of spurious signals is to be achieved under unfavorable transfer conditions. The principles of the spread spectrum technique are described for example in the xe2x80x9cTaschenbuch der Hochfrequenztechnikxe2x80x9d, 5th edition 1992, publ. Springer-Verlag, Chapter 0 46 ff. It is also indicated there that this technique was used hitherto in particular in the field of air and space travel for the transfer of information under particularly difficult conditions. Also in fields in which a particular protection against interception is involved, for example in radar technolgy and navigation, this spread spectrum technique was frequently used. A type of spread spectrum technique is the so-called direct sequence method (DS method), in which the signal which is to be transferred, which is present in digital form, is modulated with a PN code sequence (in which PN stands for xe2x80x9cpseudo Noisexe2x80x9d), which is provided so that a spreading of the signal which is to be transferred is achieved.
Here, the possibility exists of modulating the data signal to be transferred before spreading onto a carrier and then spreading the modulated carrier or else only spreading the data signal to be transferred and then modulating the spread signal onto the carrier. On the receiver side, the data signal is recovered by the spread signal being unspread again by mixing in the PN code sequence. This unspreading process is carried out using an autocorrelation stage. The PN code sequences used in the spread spectrum transfer methods hitherto have a very great length and are generally produced with binary shift registers. For this, a relatively great circuitry expenditure is necessary, so that the spread spectrum technique has not been used hitherto in data transfer systems which are to be realized expending as little cost as possible.
Increasingly more data transfer systems are required which are insensitive to interference, which are to be produced in large numbers and at low costs. Examples of this are systems by which the data picked up from water meters, gas meters, electricity meters or calorimeters on radiators can be transferred via radio to reading apparatus or to central storage apparatus, so that the expenditure for drawing up calculations of consumption can be reduced. The electronic systems used in the said counters are battery-operated, so that in addition to the low costs also a low energy consumption represents one of the preconditions for their general applicability. The requirement of the low energy consumption only permits short data telegrams, care having to be taken, however, that nevertheless a problem-free evaluation, not adulterated by interference, is made possible of the transferred information.
The invention is therefore based on the problem of providing a method of the type initially mentioned, by means of which favorably-priced apparatus can be constructed, which nevertheless permit a transfer of data which is insensitive to interference.
According to the invention, this problem is solved in that with the method initially mentioned, a periodic rectangular signal with a constant duty cycle is used as the PN code sequence.
The method according to the invention makes possible the production of the PN code sequence by means of a simple rectangular generator, i.e. without a great switching expenditure. In addition, the use of this particular PN code sequence makes possible a very quick synchronizing of the generator which on the receiver side produces the PN code sequence which is required for the unspreading of the data signal. This very short synchronizing time permits a considerable shortening of the overall length of the transmitted data telegram, so that the target which is aimed for, namely of energy-saving data transfer insensitive to interference, is achieved.
Advantageously, the duty cycle of the PN code sequence is set at 1:4, which produces a favorable energy distribution in the spectrum of the spread signal and makes possible a quick synchronizing in the autocorrelation stage of the receiver.
An embodiment of the invention is now explained by way of example with the aid of the drawing, in which: